A bottle of fine booze (whisky is common, wine works too) is a nice gift but I do recommend adding a box of chocolates or fruit basket that he can share with his staff. On top of that, a formal letter to the hotel GM citing excellent service without calling out the specifics is also helpful.

And as arlflyer indicated, please do not give cash. This is not the bellhop or doorman so the usual handshake tip is probably not appropriate. Large amounts sent independently risks getting him in trouble and/or make him feel uncomfortable, even insulted (what if he really was just being a great employee taking care of Diamonds with the unsold premium product?).